Hybrid vehicles are typically powered by both an internal combustion engine (ICE) and an electric motor. Hybrid vehicles provide a longer operating range than purely electric vehicles while emitting fewer pollutants than purely ICE powered vehicles.
In what is commonly known as a series hybrid vehicle, the ICE is used to turn a generator. The generator produces electricity that is used to power the electric motor or charge the batteries. The electric motor drives the transmission, which in turn drives the wheels. As such, in a series hybrid vehicle, the ICE is only used to produce electricity.
Another type of hybrid vehicle is what is commonly known as a parallel hybrid vehicle. Typically, in this type of hybrid vehicle, both the ICE and the electric motor drive the transmission, which in turn drives the wheels. Depending on the operating condition, only the electric motor can drive the transmission, only the ICE can drive the transmission, or both the ICE and the electric motor can drive the transmission together.
A majority of hybrid vehicles being manufactured are automobiles. However, the advantages of a hybrid propulsion system described above would also apply to vehicles such as motorcycles and three-wheel straddle-type vehicles. These vehicles typically use what is known as a sequential transmission. In such a transmission the various shift positions, also known as gears, are selected in sequence. Thus, in order to get to the third gear from the first gear, the second gear has to be selected before selecting the third gear. This eliminates shifting mistakes that could occur with an automotive manual transmission. The shifter arrangement is also simplified since the driver only has to select between a gear up and a gear down position rather than moving a stick through an H pattern typical of manual transmission. The shifter arrangement is also more compact than that of a manual transmission. Due to these advantages, this type of transmission is also being used in racecars and is beginning to appear in high-end cars.
As such it would be advantageous to combine the benefits of a sequential transmission with those of a hybrid propulsion system.
One of the issues associated with combining a sequential transmission with a parallel hybrid propulsion system arises when the transmission is driven by the electric motor only and the ICE then needs to be powered to assist the electric motor. When the transmission is driven by the electric motor only, the transmission, or the portion of the transmission connecting the ICE to the output shaft of the transmission, is in a neutral position such that the input shaft of the transmission to which the ICE is connected does not turn. When the ICE needs to be used to power the transmission, the transmission has to be moved to a shift position corresponding to a current operating speed of the vehicle (i.e. the shift position at which the transmission would be had the vehicle been operating with the ICE). However, due to the sequential nature of the transmission, the transmission needs to go through the lower shift positions (i.e. low gears) before reaching the required shift position. Since there is a large speed difference between the output shaft of the transmission and that of the input shaft, when engaging the first gear following the neutral position, there is a rapid acceleration of the input shaft, which produces noise and puts substantial stress on the transmission. If the speed difference between the output shaft of the transmission and that of the input shaft is too large, then it is possible that the shift position cannot be engaged.
Therefore, there is a need for a sequential transmission that can be used in a hybrid propulsion system for a vehicle and of a drivetrain having such a transmission.